1. Field of the Invention
The present invention relates to a pressing speed controller for a hydraulic press for compression-molding a substance to be molded.
2. Description of the Related Arts
In manufacturing a substance to be molded, for example, a front panel, a body panel or the like of an automobile by compression-molding a thermosetting resin made seat material, that is, a SMC (Sheet Molding Compound) or FRP, there has been used such a hydraulic press as shown in FIG. 2.
A press 40 includes a base 41; uprights 42 erected on the base 41; a crown 43 provided on the upper portions of the uprights 42; a hydraulic cylinder 44 provided on the crown 43; a slide 46 supported at the lower end of a plunger 45 of the hydraulic cylinder 44 and guided by the uprights 42 so as to be movable up and down; a fixed mold 47 exchangeably fixed on the upper surface of the base 41; a movable mold 48 exchangeably mounted on the lower surface of the slide 46; an oil tank 50 for supplying a working oil to the hydraulic cylinder 44 through an oil filler valve 49; and a hydraulic circuit 51 for controlling the supply of a pressing oil.
The above hydraulic circuit 51 includes a variable capacity pump 54 connected to the hydraulic cylinder 44 by an oil tube 53 through a solenoid proportional flow control valve 52; a pressing speed setting means 55 and an amplifier 56, and a relief valve 57 provided between the pump 54 and the valve 52.
This hydraulic circuit 51 functions to change the opening of the solenoid proportional flow control valve 52 in proportion with the pressing speed set by the setting means 55.
Namely, as shown in FIG. 3, in the whole stroke extending from the descending to the ascending of the position of the slide 46, that is, the movable mold 48, the descending speed of the plunger 45 of the hydraulic cylinder 44 is controlled in a multi-stage for the times (t.sub.1, t.sub.2, t.sub.3) when the movable mold 48 is descended from the top dead point and the clamping is completed. In the above, conventionally, there has been performed an open loop control for controlling the pressing descending speed at the period of t.sub.3 (charging time) shown in FIG. 3 by changing the opening of the solenoid proportional flow control valve 52 in proportion with the above setting.
However, the above conventional technique has the following disadvantage: Namely, in a low speed pressing descending region t.sub.3 (2 mm/sec or less), when a load pressure (resin pressure) is increased with respect to the oil amount controlled by the solenoid proportional flow control valve 52 for controlling the pressing speed of the movable mold 48, that is, the plunger 45 (open loop), the amount of oil leaked from the hydraulic cylinder side 44 (the oil filler valve 49, sub-cylinder, oil valve and the like) is increased. Consequently, the real descending speed is made slow with respect to the setting speed, and further, is not increased up to the set-up pressure of a pressure change-over switch thereby obstructing the change-over from the pressing speed control to the pressure control.